Thermal interface wafer and method of making and using the same

ABSTRACT

A thermal interface wafer for facilitating heat transfer from an electronic component to a heat sink. The wafer is formed from at least one elongate, vertically-oriented strip of thermally conductive material having a layer of conformable, heat-conducting material formed thereon. Preferably, the substrate comprises a metal foil, such as aluminum or some other thermally-conductive metal, that is formed as a flat, spiral-like coil. Such strip may further be configured to have a serpentine configuration, or may alternatively be formed from a multiplicity of strips. The present invention further provides for methods of transferring heat from an electronic component to a heat sink, as well as methods for fabricating the thermal interface wafers of the present invention.

CROSS-REFERENCE TO R ELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 09/876,763, filed Jun. 7, 2001 entitled Thermal Interface now in theissuance process, now U.S. Pat. No. 6,672,378.

BACKGROUND OF THE INVENTION

Methods and materials for transferring heat across the interface betweena heat-dissipating component, which typically includes variouselectronic components in semi-conductor devices, to an external heatdissipater or heat sink are well-known in the art. In this regard, theelectronic component generates substantial heat which can cause thecomponent to fail catastrophically. Even to the extent the componentdoes not fail, such elevated temperatures can and frequently do affectthe component's electrical characteristics and can cause intermittent orpermanent changes. Indeed, the life of an electronic component isdirectly related to its operating temperature, and a temperature rise ofso much as 10° C. can reduce the component's life by 50%. On the otherhand, a corresponding decrease in 10° C. can increase a component's lifeby 100%.

According to contemporary methodology, the typical solution to such heatdissipation problems is to provide an external heat dissipater or heatsink coupled to the electronic device. Such heat sink ideally provides aheat-conductive pathway from the heat dissipating component to outwardlyextending structures such as fins or other protuberances havingsufficient surface area to dissipate the heat into the surrounding air.To facilitate such heat dissipation, a fan is frequently utilized toprovide adequate air circulation over the fins or protuberances.

However, essential to any effective system for removing heat from anelectronic component to a heat sink requires efficient and uniform heattransfer at the interface between the component and the heat sink. Amongthe more efficient means by which heat is transferred across theinterface between the component and the heat sink has been the use ofheat conductive pads. Such heat conductive pads are typically pre-formedto have a shape or footprint compatible with a particular electroniccomponent and/or heat sink, such that a given pad may be easily appliedthereto prior to coupling the heat sink to the electronic component.

Exemplary of such contemporary phase change pad-type thermal interfaceproducts are THERMSTRATE; ISOSTRATE and POWERSTATE (each registeredtrademarks of Power Devices, Inc. of Laguna Hills, Calif.). TheTHERMSTRATE interface comprises thermally conductive, die-cut pads whichare placed intermediate the electronic component and the heat sink so asto enhance heat conduction there between. The THERMSTRATE heat padscomprise a durable-type 1100 or 1145 aluminum alloy substrate having athickness of approximately 0.002 inch (although other aluminum and/orcopper foil thickness may be utilized) that is coated on both sidesthereof with a proprietary thermal compound, the latter comprising aparaffin base containing additives which enhance thermal conductivity,as well as control its responsiveness to heat and pressure. Suchcompound advantageously undergoes a selective phase change insofar asthe compound is dry at room temperature, yet liquefies just below theoperating temperature of the great majority of electronic components,which is typically around 50° C. or higher, so as to assure desired heatconduction. When the electronic component is no longer in use (i.e., isno longer dissipating heat), such thermally conductive compoundresolidifies once the same cools to room temperature.

The ISOSTRATE thermal interface is likewise a die-cut mounting pad andutilizes a heat conducting polyamide substrate, namely, KAPTON (aregistered trademark of DuPont) type MT. The ISOSTRATE thermal interfacelikewise is a proprietary paraffin-based thermal compound utilizingadditives to enhance thermal conductivity and to control its response toheat and pressure.

Additionally exemplary of prior-art thermal interfaces include thosedisclosed in U.S. Pat. No. 5,912,805, issued on Jun. 15, 1999 to Freuleret al. and entitled THERMAL INTERFACE WITH ADHESIVE. Such patentdiscloses a thermal interface positionable between an electroniccomponent and heat sink comprised of first and second generally planarsubstrates that are compressively bonded to one another and have athermally-conductive material formed on the outwardly-facing opposedsides thereof. Such interface has the advantage of being adhesivelybonded into position between an electronic component and heat sink suchthat the adhesive formed upon the thermal interface extends beyond thejuncture where the interfaces interpose between the heat sink and theelectronic component.

Exemplary of the processes for forming thermal interfaces according tocontemporary methodology include the teachings set forth in U.S. Pat.No. 4,299,715, issued on Nov. 10, 1981 to Whitfield et al. and entitleda METHODS AND MATERIALS FOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTSAND THE LIKE; U.S. Pat. No. 4,466,483, issued on Aug. 21, 1984 toWhitfield et al. and entitled METHODS AND MEANS FOR CONDUCTING HEAT FROMELECTRONIC COMPONENTS AND THE LIKE; and U.S. Pat. No. 4,473,113, issuedon Sep. 25, 1984 to Whitfield et al., and entitled METHODS AND MATERIALSFOR CONDUCTING HEAT FROM ELECTRONIC COMPONENTS AND THE LIKE, thecontents of all three of which are expressly incorporated herein byreference.

In addition to the construction of thermal interfaces, there havefurther been advancements in the art with respect to the thermalcompositions utilized for facilitating the transfer of heat across aninterface. Exemplary of such compounds include those disclosed in U.S.Pat. No. 6,054,198, issued on Apr. 25, 2000 to Bunyan et al. andentitled CONFORMAL THERMAL INTERFACE MATERIAL FOR ELECTRONIC COMPONENTS,and U.S. Pat. No. 5,930,893, issued on Aug. 3, 1999 to Eaton andentitled THERMALLY CONDUCTIVE MATERIAL AND METHOD OF USING THE SAME, theteachings of which are expressly incorporated by reference.

Such compositions, along with the aforementioned pad-type thermalinterfaces, however, are each intended to be applied or positioned in aflat, horizontal plane (i.e., an X/Y axis) that runs parallel betweenthe electronic component and heat sink. As a consequence, heat must passthrough such materials via a parallel horizontal plane. As iswell-known, however, the ability of a material to conduct heat istypically lower across a generally parallel or horizontal cross-sectionof material than could be attained through the same material maintainedin a generally perpendicular or vertical orientation (i.e., a Z axis).

Notwithstanding the increased thermal conductivity along the verticalaxis, contemporary methodology predominately emphasizes a thermalinterface construction that is as thin as possible and/or utilizes aminimal amount of layers that are present between the heat sink andelectronic component. Accordingly, there has not yet been available anytype of thermal interface which concomitantly possesses athermally-conductive material or substrate disposed in a verticalorientation (i.e., perpendicular relative the electronic component andheat sink) that additionally is thin enough to optimally facilitate thetransfer of heat from the electronic component to a heat sink. There isalso lacking any such type of thermal interface that can be readilyfabricated from well-known, thermally-conductive materials that can bereadily deployed in virtually all types of heat transfer applicationsrequiring the dissipation of heat from an electronic component to a heatsink. Still further, there is lacking any type of thermal interface ofthe aforementioned variety that is easy to handle and utilize, effectivein filling voids between and transferring heat away from a givenheat-dissipating component to a heat sink, is easy and relativelyinexpensive to produce, and does not require special handling.

BRIEF SUMMARY OF THE INVENTION

The present invention specifically addresses and alleviates theaforementioned deficiencies in the art. In this regard, the presentinvention is directed to a thermal interface comprised of a wafer orwafer-type structure positionable between a heat sink and electroniccomponent for facilitating the transfer of heat thereacross. The waferis comprised of at least one strip of a thermally-conductive substratethat is maintained in a generally vertical or perpendicular orientationalong a horizontal plane that, in use, is interposed between theinterface mating surfaces of the electronic component and heat sink. Thevertically oriented strip defines first and second sides upon at leastone of which is formed a layer of conformable heat-conductive material.With respect to the latter, the same preferably comprises a phase-changematerial that is operative to remain solid at room temperature, butliquify or become molten when subjected to the elevated temperaturesassociated with the operation of the electronic component. Preferably,such heat-conductive materials will be formulated to undergo a phasechange at 51° C. or higher, to thus ensure ideal mechanical contactbetween the electronic component and heat sink just prior to when mostelectronic components attain the temperature at which they are intendedto operate.

The substrate will preferably comprise an elongate strip of metal foil,such as copper, gold, silver or aluminum, with aluminum being mostideal. Such substrate strip may be formed as a tightly-wound coil thatassumes a generally spiral-like configuration that extends outwardlyalong a horizontal axis. In an alternative embodiment, the elongatestrip of substrate may be formed such that the same assumes a serpentineconfiguration. Still further, such wafer may be comprised from amultiplicity of strips of thermally-conductive substrate withthermally-conductive material formed thereon that are arranged inelongate rows in generally parallel relation to one another. In allembodiments, however, the wafer defining the thermal interface willconsist of alternating vertically or perpendicularly-oriented segmentsof thermally-conductive substrate and thermally-conductive materialrelative the electronic component and heat sink coupled therewith.

The present invention further comprises methods of fabricating thethermal interface wafers of the present invention. According to apreferred embodiment, the method comprises the initial step of providinga sheet of the thermally-conductive substrate and applying or forming alayer of heat-conductive material thereon, the latter of whichpreferably comprises any of a variety of known thermally-conductivematerials possessing selective phase-change properties. The sheet withthermally-conductive material formed thereon is then rolled to a desiredconfiguration such that the same defines a specified cross-sectionalshape that conforms to the shape of a particular interface. For example,such sheet may be rolled such that the same assumes a generallycylindrical configuration such that a circular cross-section is definedthereby or rolled as a block such that a square or rectangularcross-sectional shape is defined thereby. Such rolled sheet of materialis then sliced to a specified thickness, which preferably does notexceed 0.2″, and preferably is 0.1″ or less. In an optional step, suchsliced wafer may be compressed horizontally or flatwise to thus make thesame more durable, compact and insure better contact between thecomponent and heat sink.

The resultant thermal interface wafer produced may then be utilized asper conventional thermal interfaces. Specifically, such wafer may beinterposed directly between a heat sink and electronic component tofacilitate the transfer of heat thereacross. In some applications, suchwafer may be compressed further or clamped between electronic componentand heat sink to ensure an ideal, secure engagement at the interfacetherebetween.

The present invention thus provides a thermally-conductive interfacewafer that is substantially more effective than prior art interfaces infacilitating the transfer of heat away from an electronic component to aheat-dissipating component, such as a heat sink.

The present invention further provides a thermally-conductive interfacewafer that can be readily fabricated from existing materials and readilydeployed for use in a wide variety of thermal interface applications.

The present invention further provides a thermally-conductive interfacewafer that is operative to provide a highly efficient heat transfermedium, which offers substantial economic advantages for options ofeliminating costly heat-dissipating mechanisms or towards the reductionin size, weight and cost of heat sinks.

The present invention further provides a thermally-conductive interfacewafer which, in use, is operative to fill voids or gaps present at theinterface between the heat sink and any electronic component to ensurebetter mechanical contact, and hence thermal conductivity thereacross.

The present invention still further provides a method for manufacturingthe novel thermally-conductive interface wafers of the presentinvention, as well as utilizing the same to transfer heat from aheat-dissipating component and a heat sink.

BRIEF DESCRIPTION OF THE DRAWINGS

These, as well as other features of the present invention, will becomemore apparent upon reference to the drawings wherein:

FIG. 1 is an exploded perspective view of a heat sink positioned forattachment to electronic component further showing a thermal interfacewafer being disposed therebetween, the wafer being constructed inaccordance with an embodiment of the present invention;

FIG. 2 is an elevated perspective view of the thermal interface waferdepicted in FIG. 1;

FIG. 3 is a cross-sectional view taken along the line 3—3 of FIG. 2;

FIG. 4 is a perspective view of a rolled sheet of a thermally-conductivesubstrate with a layer of heat-conformable material formed thereon, therolled sheet being oriented for cross-sectional slicing therethrough;and

FIG. 5 is a flow chart diagram illustrating the steps for fabricatingthe thermal interface wafers of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The detailed description set forth below is intended as a description ofthe presently preferred embodiment of the invention, and is not intendedto represent the only form in which the present invention may beconstructed or utilized. The description sets forth the functions andsequences of steps for constructing and operating the invention. It isto be understood, however, that the same or equivalent functions andsequences may be accomplished by different embodiments and that they arealso intended to be encompassed within the scope of the invention.

Referring now to the drawings, and initially to FIG. 1, there is shown athermal interface 10 constructed in accordance with the presentinvention. The thermal interface 10 is specifically designed andconfigured to facilitate the transfer of heat away from an electroniccomponent 12 to a heat sink 14. In this regard, the thermal interface 10is specifically designed and adapted to be interposed between theelectronic component 12 and heat sink 14 as per conventional interfacestructures and compositions. As is well known, most heat sinks such as14, are provided with outwardly-extending structures, such as elongatemembers 14 a, having sufficient surface area to dissipate the heatgenerated from the component 12, across the interface 10, and into thesurrounding air. To help achieve that end, a fan or other like device(not shown) is typically deployed to facilitate adequate air circulationover such extensions 14 a.

Preferably, the thermal interface 10 comprises a wafer or wafer-likestructure formed to have a shape or footprint compatible with theparticular electronic component and/or heat sink utilized therewith tothus enable the thermal interface 10 to meet or exceed surface areacontact at the juncture between the electronic component 12 and heatsink 14. As is well-known in the art, to maximize the ability of heat tobe transferred across an interface, mechanical contact is preferablymaintained at all points about the juncture between the component,thermal interface, and heat sink.

As shown in both FIGS. 1 and 2, the thermal interface is preferablyformed from an elongate strip or substrate 18 coiled about itself in agenerally spiral-like fashion along a horizontal plane. Such strip orsubstrate comprises a thin, malleable strip of thermally-conductivematerial, and preferably will comprise a metal foil having a high degreeof thermal conductivity. Among the preferred metallic foils for use inthe practice of the present invention include copper, gold, silver andaluminum, with aluminum being exceptionally preferred due to its lowcost and high degree of thermal conductivity. Such strip will furtherpreferably have a thickness of 2 mil or less.

Such substrate will preferably have a width, represented by the letter Ain FIG. 2 of approximately 0.02″ or less. Preferably, such thicknesswill be 0.01″ or less and, according to a more highly referredembodiment, between 0.005 to 0.01″. The substrate strip will furtherpreferably have formed upon at least one respective side thereof a layerof a thermally conductive compound 20 formulated to facilitate andenhance the ability of the interface 10 to transfer heat away from theelectronic component to the heat sink. Such thermally conductivecompound 20 may take any of a variety of compositions well known tothose skilled in the art, such as thermal grease and the like.Preferably, such layer is formulated to have selected phase-changeproperties such that the compositions is substantially solid at roomtemperature (i.e., when the electronic device is not operating), butviscous or liquid when the electronic component attains its normaloperating temperature. As is well-known, becoming liquid or viscous atsuch elevated temperature enables the composition to fill any voids orgaps formed by surface irregularities at the interface to become filled,thereby maintaining a generally continuous mechanical contact for heatto transfer from the component, across the interface and to the heatsink. Following operation of the electronic component, the electroniccomponent will consequently return to room temperature and a layer ofthermal compound 20 will reassume its solid phase.

Preferably, the thermally conductive composition may take any of thosedisclosed in Applicant's co-pending patent application entitled PHASECHANGE THERMAL INTERFACE COMPOSITION HAVING INDUCED BONDING PROPERTY,filed on Apr. 12, 2001, Ser. No. not yet assigned, and Applicant'sco-pending patent application entitled GRAPHITIC ALLOTROPE INTERFACECOMPOSITION AND METHOD OF FABRICATING THE SAME, filed on May 18, 2000,and assigned application Ser. No. 09/573,508, the teachings of which areexpressly incorporated herein by reference. Such thermal compounds havethe desirable phase-change properties of assuming a solid phase atnormal room temperature, but liquify at elevated temperatures ofapproximately 51° C. or higher, which is typically just below theoperating temperatures at which most electronic components are intendedto operate. It should be understood, however, that a wide variety ofalternative thermally conductive materials and compounds are availableand readily known to those skilled in the art which could be deployedfor use in the practice of the present invention.

When the strip-type substrate with thermally-connective material formedthereon is coiled in the manner illustrated in FIGS. 1 and 2, there isthus consequently produced a horizontally-arranged row of alternating,vertically-oriented layers of the substrate 18 and thermally-conductivecomposition 20, as depicted in the cross-sectional view of FIG. 3. Inuse, when interposed between the heat sink and electronic component, theinterface wafer will be operatively maintained such that the strip 18and thermally-conductive composition 20 assume the alternating,generally vertical orientation that will be perpendicular relative theheat sink and electronic component compressed thereagainst. As will beappreciated, in alternative configurations whereby the electroniccomponent and heat sink are maintained in a generally verticalorientation, the strip 18 and thermally-conductive composition 20 willassume an alternating, generally horizontal orientation that will beperpendicular relative the electronic component/heat sink interface.

As will be appreciated by those skilled in the art, such structure is adramatic departure from prior art interface design which typicallydeploys generally planar interfaces of thermally-conductive materialsthat are applied as flat, horizontal substrates, layers or films. Quiteadvantageously, however, the vertical orientation of the substraterelative the electronic component and heat sink, coupled with thevertically-oriented layers of thermally-conductive compound disposedtherebetween causes the wafer-like interface to greatly facilitate heattransfer. In this respect, presently it has been determined that aninterface wafer having an approximate width of 0.01″ comprised of a 2mil thick aluminum foil substrate is capable of producing a thermalconductivity of approximately 1.3 W/in° C. or 51.2 W/m° K, which, aswill be appreciated in the art, is substantially greater than mostcurrently available interfaces and materials.

Although depicted in FIGS. 1 and 2 as a coiled-type structure, it willbe recognized that the strip of substrate having thethermally-conductive material formed thereon may be shaped and formed totake any of a variety of configurations which will consequently producethe desired alternating, vertically-disposed cross-sectional patterndepicted in FIG. 3. For example, although not shown such strip withthermally-conductive compound formed thereon may be formed to assume agenerally serpentine configuration or, alternatively, may simplycomprise a plurality of elongate substrate strips that are arranged ingenerally parallel relation to one another along a common horizontalplane. With respect to the latter configuration, it is contemplated thatsuch multiplicity of substrate strips will be compressed to one anotherto thus assume the cross-sectional configuration depicted in FIG. 3.Accordingly, it will be recognized that any manner by which thethermally-conductive strip with thermally-conductive composition formedthereon can assume and maintain a general vertical orientation that isperpendicular to the heat sink and electronic component will accomplishthe objectives of the present invention.

With respect to the manner by which the wafer-type interfaces of thepresent invention may be fabricated, there is depicted in FIG. 5 a flowchart of the steps of a method 22 to form the same. Initially, there isprovided a sheet of the thermally-conductive substrate 24 which, asdiscussed above, will preferably comprise a metal foil. The layer ofthermally-conductive material is then formed upon a respective side ofthe sheet, via step 26. The sheet with thermally-conductive materialformed thereon is then tightly rolled via step 28. The resultant roll isthen sliced 30 to a specified thickness to produce the resultant thermalinterface wafer.

With respect to the latter step, there is perspectively illustrated inFIG. 4 the contemplated manner by which a rolled sheet 32 of substratewith thermally-conductive material formed thereon will be configured andoriented in order to be cut, via knife blade 34, to produce the thermalinterface wafers of the present invention. It will be readilyrecognized, however, that such roll can be cut through such methods assawing, grinding, and possibly laser cutting. Other methods known in theart my also be utilized. Along these lines, it is contemplated that thesubstrate sheet with thermally-conductive compound formed thereon 32 maybe selectively rolled such that the cross-sectional cuts madetherethrough will attain not only the desired thickness, but surfacearea sufficient to facilitate the conduction of heat across a giveninterface between electronic component and heat sink. For example, it iscontemplated that the substrate sheet may be rolled to assume agenerally cylindrical configuration, such that a circular cross-sectionis defined thereby, or rolled to assume a block form, to thus form arectangular or square-like cross section as shown.

As discussed above, the wafers produced by slicing the rolled substratesheet will preferably be such that the wafers have a thickness of 0.2″or less, and preferably 0.1″ or less. It is further contemplated that inorder to minimize thickness of the wafer, as well as the upper and lowersurfaces thereof for interposition between a heat sink and electroniccomponent, the resultant wafer may be compressed or flattened further byapplying a horizontal or flatwise (i.e., along an X/Y axis) compressiveforce of approximately one hundred pounds per square inch or lessthereto. Alternatively, such wafer may be simply clamped into positionbetween the heat sink and electronic component to help ensure idealmechanical contact between the electronic component and heat sink.Otherwise, it is contemplated that the thermal interface may be utilizedas per conventional thermal interfaces whereby the same is simplyinterposed between the heat sink and the electronic component with theelectronic component being allowed to operate as normally intended. Asdiscussed above, due to the vertical orientation of the substrate withheat conformable material, there will thus advantageously be providedsuperior thermal conduction.

Additional modifications and improvements of the present invention mayalso be apparent to those of ordinary skill in the art. Thus, theparticular combination of parts and steps described and illustratedherein is intended to represent only certain embodiments of the presentinvention, and is not intended to serve as limitations or alternativedevices and methods within the spirit and scope of the invention.

1. A method for facilitating the transfer of heat from an electroniccomponent to a heat sink across an interface therebetween, the methodcomprising the steps: a) providing a thermal interface waferinterposable between said electronic component and said heat sink, saidwater comprising at least one elongate vertically-oriented stripsubstrate having first and second surfaces, said substrate having atleast one layer of a phase-change, heat-conductive material formed upona respective surface thereof, said heat-conductive material beingformulated to enhance the heat transfer from said electronic componentto said heat sink; b) interposing the thermal interface wafer of step a)between said electronic component and said heat sink such that thesubstrate of such wafer assumes a substantially perpendicularorientation relative said electronic component and said heat sink; c)compressively engaging said electronic component to said heat sink withsaid thermal interface disposed therebetween; and d) operating saidelectronic component such that heat is generated thereby, said heatbeing sufficient to cause said phase-change, heat-conductive material instep a) to transition from a solid phase to a liquid phase.
 2. Themethod for facilitating the transfer of heat of claim 1 wherein in step(a), said substrate comprises a thermally conductive metal foil.
 3. Themethod of claim 2 wherein said foil is formed from the group consistingof copper, gold, silver and aluminum.
 4. The method of claim 1 whereinsaid layer of conformable, heat-conducting material is formulated tohave a melting point of approximately 51° C.
 5. The method of claim 1wherein in step a), said thermal interface wafer has a generallyrectangular configuration.
 6. The method of claim 1 wherein in step a),said thermal interface wafer has a generally square configuration.